The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode of a vehicle, reverse excitation mode, drag mode at release of an accelerator pedal of a vehicle, and mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
As shown in FIG. 9, in the drag mode clutch current flowing in a coil of the electromagnetic clutch is cut off when the vehicle speed is below a predetermined speed V.sub.4 to prevent stalling of the engine. When the accelerator pedal is released and the vehicle speed becomes lower than a still lower vehicle speed V.sub.2, a small drag current flows in the coil, thereby applying a small drag torque to a driven member of the clutch to reduce clearances (play) in the transmission.
Although clutch current of the electromagnetic clutch is constant, the clutch torque is not determined only by the clutch current. The clutch torque is influenced by engine speed and slipping of the clutch. An experimentally established characteristic of clutch torque depending on the engine speed is shown in FIG. 7. Namely, clutch torque increases as the engine speed rises until the engine speed reaches a certain value, for example, 1500 rpm and gradually decreases thereafter. Consequently, even though the clutch control variable drag current I.sub.D is constant, drag torque T.sub.D varies depending on the engine speed as shown for example by a chain line in FIG. 9. Thus, drag torque can not be accurately controlled.